EGR has long been provided to engines to control NOx emissions and reduce engine pumping work. EGR can lower engine pumping work by raising intake manifold pressure. EGR can lower NOx emissions by reducing peak in-cylinder combustion temperatures. More recently, EGR systems have started to cool EGR to further reduce in-cylinder combustion temperatures, thereby further reducing engine NOx emissions. However, cooled EGR is not necessary at all engine operating conditions and therefore some EGR systems provide both unconditioned and cooled EGR.
Although EGR systems can help to lower engine emissions and improve engine operation by providing both unconditioned and cooled EGR, such systems also provide challenges as to determining whether or not the system is operating as is desired. For example, it may be difficult under some conditions to determine whether or not control valves are positioned as is desired to provide the requested cooled or unconditioned EGR. Some EGR systems include a model against which measured engine operating conditions are compared so as to determine whether or not the EGR system is operating as is desired. However, adjusting a position of a two state cooler control valve can cause discontinuities in some modeled systems so that it may be difficult to assess whether or not operation of the EGR system is degraded. If the discontinuities caused by switching a cooler control valve are not properly processed, false positive or false negative indications of EGR system degradation may result.
The inventor herein has recognized the above-mentioned disadvantages and has developed a method for monitoring an EGR system. One example of the present description includes a method for monitoring an EGR system, comprising: indicating EGR system degradation in response to a first EGR gas temperature model when a valve is commanded to a first position; and indicating EGR system degradation in response to a second EGR gas temperature model when the valve is commanded to a second position.
By providing separate and different models of EGR system behavior for each valve position, it is possible to better estimate EGR system behavior at least during some conditions. In one example, a model is provided to estimate EGR gas temperature downstream of an EGR cooler when a valve is commanded to a position where EGR gas is directed to the EGR cooler. A second model is provided to estimate EGR gas temperature downstream of the EGR cooler when the valve is commanded to a second position bypassing the EGR cooler. By providing separate models of different portions of the EGR system, it is possible to limit the effect that valve switching may have on individual models since the models can continue to estimate system parameters independent of valve position.
The present description may provide several advantages. In particular, the approach can improve model based parameter estimates since discontinuities that may be caused by switching a two state valve may not significantly influence the estimates provided by separate models. Further, the approach may be simpler to implement since all system dynamics do not have to be integrated into a single model to arrive at a desired model output. Further still, the present approach may simplify model calibration.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.